1. Field of the Invention
The present invention relates to a semiconductor device such as light-emitting and light-receiving elements serving as basic circuit elements in optoelectronics and such as a transistor and, more particularly, to a semiconductor device which can be used as any one of a light-emitting element, a light-receiving element, and a transistor.
2. Description of the Related Art
As shown in FIG. 1, a conventional light-emitting element has a structure in which an n- or p-type cladding layer 2, an active layer 3 consisting of undoped GaAs, and a p- or n-type cladding layer 4 are sequentially stacked on an n- or p-type substrate 1, and ohmic electrodes 5 and 6 are formed on both surfaces of the resultant multi-layered structure. That is, the active layer 3 is sandwiched between the two cladding layers 2 and 4. Note that the refractive index of the active layer 3 is larger than that of each of the cladding layers 2 and 4, and that the band gap of each of the cladding layers 2 and 4 is larger than that of the active layer 3.
When a forward bias is applied to the light-emitting element shown in FIG. 1, electrons and holes are injected from the cladding layers 2 and 4 to the active layer 3. The injected electrons and holes are confined in the active layer 3 having a smaller band gap than that of the cladding layer 2 or 4 and are recombined to emit light. The emitted light is confined in the active layer having a larger refractive index than that of the cladding layer 2 o 4 and propagates in a direction perpendicular to the drawing surface. The light emerges from the end face of the element.
As shown in FIG. 2, a conventional light-receiving element has a structure in which an n- or p-type epitaxial layer 7, an i-type layer 8 consisting of undoped GaAs, and a p- or n-type epitaxial layer 9 are sequentially stacked on an n- or p-type substrate 1, and ohmic electrodes 5 and 6 are formed on both surfaces of the multi-layered structure. That is, the i-type layer 8 is sandwiched between the two epitaxial layers 7 and 9.
When light having an energy larger than the band gap of the i-type layer 8 of the light-receiving element shown in FIG. 2 is incident on the i-type layer 8, electron-hole pairs are generated in the i-type layer 8 and flow into the epitaxial layers 7 and 9. When the amount of incident light is increased, the number of electron-hole pairs is increased accordingly. A current capable of being extracted across two ends of the light-receiving element is increased. Therefore, since a current corresponding to the amount of incident light flows, optical detection can be performed.
As is apparent from comparison between the light-emitting element shown in FIG. 1 and the light-receiving element shown in FIG. 2, the light-emitting and light-receiving elements have the same basic structure. Therefore, it may be possible to obtain light-emitting and light-receiving elements by using identical elements.
In order to form an inversion distribution in the active layer of the light-emitting element, carriers (electrons and holes) having a concentration of 10.sup.18 cm.sup.-3 or more must be confined in the active layer. For this purpose, the carrier concentration of the cladding layer serving as a carrier source must be maximized (i.e., 10.sup.18 cm.sup.-3 or more). At present, however, n-type AlGaAs can have a maximum carrier concentration of about 4.times.10.sup.18 cm.sup.-3, and p-type GaAs can have a maximum carrier concentration of about 2.times.10.sup.19 cm.sup.-3. In order to assure a high carrier concentration, a high-concentration donor or acceptor impurity must be doped in AlGaAs. When an excessive amount of an impurity is doped in this material, crystal detects and the like occur in AlGaAs, and a high carrier concentration cannot be obtained. Therefore, it is difficult to obtain a higher carrier concentration in the state-of-the-art techniques.
Since carriers injected from the cladding layer having a carrier concentration of 10.sup.18 cm.sup.-3 to 10.sup.19 cm.sup.-3 to the active layer have a concentration of 10.sup.18 cm.sup.-3 to 10.sup.19 cm.sup.-3 or less, the active layer must be made thin (i.e., the volume of the active layer must be small) to form an inversion distribution. An active layer having a thickness of about 1,000 .ANG. has been frequently used to cope with this.
On the other hand, in the i-type layer (corresponding to the active layer of the light-emitting element) serving as the light-receiving portion of the light-receiving element, the number of electron-hole pairs must be maximum to increase an optical detection sensitivity. When the light-emitting element is directly used as a light-receiving element, the optical detection sensitivity is undesirably decreased because the thickness of the i-type layer is as small as 1,000 .ANG..
When light-emitting and light-receiving elements are formed on a single substrate in the same process, i.e., when the light-emitting and light-receiving elements are integrally formed, laser oscillation does not occur if the thickness of the active layer (i-type layer) is small. To the contrary, when the thickness of the active or i-type layer is decreased, the reception sensitivity is decreased. As a result, practical light-emitting and light-receiving elements cannot be obtained.
FIG. 3 shows a structure of a typical field effect transistor (FET). Referring to FIG. 3, n.sup.+ - or p.sup.+ -type regions 13 are formed in the surface region of an n- or p-type active layer 12 formed on a semi-insulating substrate 11. Source (S) and drain (D) electrodes 14 and 15 are in ohmic contact with the n.sup.+ - or p.sup.+ -type regions 13. On the other hand, a gate (G) electrode 16 is in Schottky contact with the n- or p-type active layer 12, and therefore a depletion layer 17 is formed in the active layer 12.
When a gate (G) voltage is changed in a state wherein a current flows from the drain (D) electrode 15 to the source (S) electrode 14, the width of the depletion layer under the gate (G) electrode 16 is changed. Therefore, a current flowing through the drain (D) and the source (S) is changed (modulated or amplified) in accordance with a change in gate (G) voltage.
As is apparent from FIGS. 1 to 3, the structures of the light-emitting and light-receiving elements are basically identical to each other but are entirely different from the transistor structure. For this reason, when an optoelectronic integrated circuit (i.e., a circuit in which an optical element and an electronic element are formed on a single substrate) is to be formed, the respective elements must be manufactured in the corresponding independent processes. In this fabrication process, an already formed element (i.e., the first element or the first and second elements) may be damaged in annealing of the second or third element. In addition, since the fabrication process is undesirably complicated, this process is not suitable for mass production.